State 1
Historic Climax Plant Community

Community 1.1
Historic Climax Plant Community

State Containing the Historic Climax Plant Community Black grama-dominated grassland: The historic plant community is dominated primarily by black grama and secondarily by dropseeds (Sporobolus flexuosus, S. cryptandrus). Because shallow caliche maintains water within the root zone of grasses longer than deeper caliche, and because caliche may store and slowly release water (Hennessy et al. 1983b), grass production is generally greater than in the Sandy site. Threeawns (Aristida spp.) occur but in lower frequency than dropseeds. Bush muhly (Muhlenbergia porteri) is less common than in the Sandy site. Winterfat (Krascheninnikovia lanata) can be the most common woody plant and soaptree yucca (Yucca elata), longleaf ephedra (Ephedra trifurca), fourwing saltbush (Atriplex canescens), and snakeweed are common. This state is defined by the capacity of black grama to persist indefinitely. Through its high basal cover, high litter cover, and consequent low rates of erosion and high infiltration rates, black grama can regenerate by both seeding and tillering. Fires may or may not be frequent (see discussion in Sandy model). If the climate hypothesis is correct, then this state may not now exist anywhere in SD-2, except during ephemeral periods suitable climate. Mesquite cannot be present in this state. Retrogression within this state caused by grazing is characterized by an increasing relative abundance of threeawns or snakeweed in most cases. Two seasons without summer rains will also lead to black grama decline (Gibbens et al., in press). Snakeweed or threeawns may become dominant within this state as long as the capacity of black grama to recover after cessation of grazing is not compromised. Gibbens and Beck (1987) and some unpublished records from the USDA-ARS Jornada Experimental Range, Las Cruces, NM provide evidence for recovery of black grama from dropseed dominance at a local scale (1 m2). Campbell and Bombarger (1934) indicate that black grama can recover within snakeweed-dominated grassland. Diagnosis: Black grama is very dominant and cover is continuous. There is evidence of black grama reproduction by seed and stolon. Large gaps (< 1 m) are very few. Litter cover is abundant. There is no mesquite. Transition to black-grama limited state (1a): Climate change (see Information sources and theoretical background) or damage to black grama and loss of soil fertility by grazing, trampling, and moderate erosion may cause this transition. Gibbens et al. (in press) note that two summers without rain will kill black grama plants, so drought in combination with selective grazing on black grama may cause its extinction. Wright and Van Dyne (1976) note that the effects of cattle trampling may be more important than the effects of grazing per se. Furthermore, those authors found an effect of grazing on only loamy sands suggesting that relatively minor variations in soil texture may determine the sensitivity of black grama to grazing. Herrick et al. (2001) suggest that loss of litter cover, plant cover, and perhaps cryptogamic crust leads to soil degradation that creates an unfavorable environment for black grama. In particular, the loss of mycorrhizal fungi required by black grama may be an important mechanism (ongoing research by Laurie Abbott, NMSU, and Jerry Barrow, USDA-ARS Jornada Experimental Range). Increases in threeawns seem to be an especially ominous indicator of a transition, although it is unclear why. According to the review by Tirmenstein (1987), Aristida purpurea is favored by winter-spring precipitation, is animal dispersed, is disturbance-adapted, and is usually unpalatable. Thus, the variety of processes postulated to reduce black grama may all favor this grass. Key indicators of approach to transition: Increases in bare ground, decreases in litter cover and black grama cover, decreased soil surface resistance to erosion, decreases in soil organic matter and microbiotic populations, increases in threeawns. Transition to the shrub-invaded state (3a): Once shrubs invade and/or begin to grow to maturity, this transition has occurred. Many investigators believe that shrub invasion is initiated by reductions in black grama, but Herbel and Gibbens (1996) suggest that mesquite invasion can occur within apparently intact black grama stands. It is possible that the latter pattern emerges when the propagule load to an intact site is very high due its proximity to a mesquite-invaded site. Alternatively, mesquite propagules may usually be present as seeds but are able to achieve maturity in the absence of fire. If the competition hypothesis is true, loss of soil stability and selective herbivory on black grama with continued grazing may promote the growth and expansion of mesquite within this state. If the fire hypothesis is true, then the reduction of fire frequency associated with the loss of fine fuels is the cause for the transition. If the small animal shrub herbivory hypothesis is true, then the elimination or reduction of mesquite seedling predators is the cause, independent of grass cover. If none of these hypotheses are true, then once introduced, mesquite may expand despite cessation of grazing. It is likely that several of these processes work in parallel or in different instances. Key indicators of approach to transition: Same as for transition 1a if the competition hypothesis is true. If the fire hypothesis is true then a reduction of black grama annual production and litter cover are indicators. If the dispersal hypothesis is true then there are no suitable indicators, other than the presence of potential seed vectors (i.e. livestock) and their connection to a seed source (a mesquite-invaded area).

State 2
Black grama Limited Grassland

Community 2.1
Black grama Limited Grassland

Additional States: Black grama-limited grassland: Communities in this state may be structurally indistinguishable from the black grama-dominated grassland. Black grama may dominate but will not show evidence of reproduction by seed and perhaps stolons. If the climate hypothesis is correct, then areas with a high cover of black grama can belong to this state. More typically, directional change via grazing or individual replacement will have already occurred, and black grama will be reduced to a subordinate component of the plant community and bunchgrasses, largely threeawns, may dominate. Snakeweed, fluffgrass (Dasyochloa pulchella) and annuals are also important components and may dominate. Snakeweed dominance seems more prevalent in this site than the Sandy site, perhaps due to the limitations imposed by the shallow petrocalcic horizon on deep-rooting shrubs. Snakeweed may achieve dominance because it is less palatable than the grasses. Furthermore, once snakeweed attains a high density, allelopathic (Tirmenstein 1999) or competitive effects may inhibit the growth of grass populations. Jameson (1970), however, failed to document any competitive suppression of snakeweed on black grama. Climate (Campbell and Bombarger 1934), fire (McDaniel et al. 2000) and beetle herbivory (Crossidius spp.; Thompson et al. 1996) may regulate patterns of snakeweed abundance. Because snakeweed is a cool-season plant, it tends to increase in response to increases in winter-spring precipitation. Diagnosis: Black grama cover is usually lower than that of bunchgrasses. There is no evidence of black grama reproduction by seed and stolon production. Mesquite is absent or rare. Transition to bunchgrass grassland (2): This is caused by the local extinction of black grama due to grazing, loss of soil fertility, drought, or other disturbance. Key indicators of approach to threshold: Decadence of black grama, pedestalling or sand burial of black grama plants, lack of reproduction. Transition to shrub-invaded grassland (4a): Mesquite propagules may be introduced to a system some time after black grama reproduction has become limited and/or black grama dominance declines. Environmental conditions are likely to be suitable for mesquite establishment within the black-grama limited state. Thus, only the presence of a mesquite-seed vector is required for this transition to take place. Key indicators of approach to threshold: There are no suitable indicators, other than the presence of potential seed vectors (i.e. livestock) and their connection to a seed source (a mesquite-invaded area). Transition to black grama-dominated grassland (1b): Black grama has been observed to survive in certain patches on the Jornada Experimental Range through the drought periods (Gibbens et al., in press). Understanding what properties distinguish these patches from areas where black grama has declined may hold important clues to preventing grassland degradation and restoring black grama. Methods for reversing the transition are currently unknown, and might require restoration of soil fertility.

State 3
Bunchgrass Grassland

Community 3.1
Bunchgrass Grassland

Bunchgrass grassland: This state is characterized by a lack of black grama and dominance by bunchgrasses (threeawns or dropseeds) or snakeweed. Diagnosis: Absence of black grama plants. Mesquite is absent or rare. Transition to shrub-invaded grassland (5a): As for transition 4a above. Transition to black grama-dominated grassland (9): Restoration techniques are unknown, but may require soil stabilization, restoration of soil fertility, and reintroduction black grama seeds.

State 4
Shrub-Invaded Grasslands

Community 4.1
Shrub-Invaded Grasslands

Shrub-invaded grasslands: Communities in this state can be distinguished from either black grama-dominated, black grama-limited, or bunchgrass grasslands by the presence of honey mesquite. In some cases, mesquite can invade otherwise healthy-looking black grama grassland that may catalyze or co-occur with events leading to black grama loss. In other cases, mesquite has invaded after significant black grama degradation has occurred and bunchgrasses may coexist with mesquite over the long term. It is believed that black grama loss (transition 6a) and possibly a transition to mesquite dunes (transition 7), eventually occurs without grazing rest and shrub control (e.g. Hennessy et al. 1983). Mesquite plants may be very small and difficult to detect. Although fire may kill small (< 1.5 yr old; Wright et al. 1976) mesquite, it is unlikely that fire frequencies will be sufficiently high to remove mesquite from a grassland once a source of mesquite propagules has been connected to a grassland. It is possible that mesquite seedlings are a normal component of black-grama dominated grassland but are suppressed by fire, small mammal herbivory, and/or competition in the black grama-dominated state (Brown and Archer 1999). Through either the exogenous effects of climate change or disturbance, or the endogenous effects of mesquite dominance, it is most common to observe a reduction in black grama reproduction coincident with mesquite increase (bunchgrasses/mesquite). There are no data available that relate grass reproduction to levels of invasion. Valentine (1936), however, indicates that beyond a height of 1-2 feet, mesquite begins to exclude grasses from around plant bases although it is unclear why. Mesquites may provide cover and nest sites for rodents (e.g. kangaroo rats) and lagomorphs (jackrabbits, cotton-tails) that increase herbivory on black grama adults and seedlings (ongoing research, Deb Peters and Brandon Bestelmeyer, USDA-ARS Jornada Experimental Range). If black grama reproduction is limited, it may be rapidly extirpated with grazing and interactions with shrubs and only bunchgrasses may remain to maintain soil stability. Twenty years after herbicidal shrub-control measures on Simona-Cruces soils, however, only minor differences in black grama cover could be attributed to the 7% reduction in mesquite cover. Diagnosis: Mesquite are present and usually conspicuous. Black grama cover may be substantial with areas around shrubs devoid of grass (Black grama/mesquite), or it may be rare to absent (bunchgrasses/mesquite). Transition to bunchgrasses-mesquite state (6): Grazing, drought, rodent activities, and competition with shrubs may drive black grama to a subordinate status or extinct. Alternatively, this transition may occur solely due to processes associated with the presence of mesquite, and thus will eventually occur unless mesquite are removed. Key indicators of approach to threshold: Decadence of black grama, pedestalling of black grama plants, lack of reproduction. Transition to mesquite shrubland state (7): This transition results in the largest change in ecosystem functioning in the site. Once grass cover is reduced below some amount and/or soil-surface disturbances due to cattle trampling reduce soil surface stability and infiltration, eolian and sheet erosion will create a transition to the mesquite shrubland state. The exposure and weathering of the petrocalcic horizon is usually observed. Key indicators of approach to threshold: Continued loss of grass cover, biotic soil crusts, and evidence of increased bare ground and erosion (e.g. the accumulation of caliche chunks and stones at the surface) indicate movement toward threshold. Pedestalling, blowouts, ripples in sand, soil surface loss will be evident. Transition to black grama-dominated, limited, or bunchgrass grassland (3b, 4b, 5b): Mesquite removal by application of herbicides, grass recovery and recovery of suitable fuel loads, and reinitiation of historic fire frequencies if the fire hypothesis is true. Attention to fuel load, continuity, and fire timing and frequency may be important if fire prevented mesquite expansion. The successful use of fire in black grama grasslands, however, depends strongly upon the size of mesquite and probably on post-fire precipitation patterns that favor black grama recovery (Drewa and Havstad 2001). At this point, it is unclear if fire can be effectively used as a management tool to promote black grama dominance. If the competition hypothesis is true, then simply reestablishing grass cover would be sufficient. Both of these approaches are unlikely to be supported in black-grama limited or bunchgrass grasslands, which generally exhibit low production and cover. If climate or mesquite seed availability alone is responsible for transitions 3a, 4a, or 5a, transitions to grasslands may be impossible to bring about.

State 5
Mesquite Shrubland

Community 5.1
Mesquite Shrubland

Mesquite shrubland: Continued reduction of grasses, or invasion of snakeweed dominated areas by mesquite, accelerates eolian and sheet erosion, reducing soil fertility in shrub interspaces and redistributing nutrients to the bases of shrubs (see the nutrient redistribution hypothesis in Sandy site; Overview: Information sources and theoretical background). Eventually, much of the soil A horizon is removed from shrub interspaces and a hardpan B horizon or petrocalcic horizon may be exposed there. Grass (or shrub) establishment on this surface may be impossible but in many cases snakeweed persists alongside mesquite shrubs. Soil accumulates around the bases of shrubs coincident with the deflation of soil in shrub interspaces. This process results in the formation of hummocks. These hummocks are not as large as those observed in the Sandy site. Dropseeds and snake weed may or may not occur in the interspaces depending upon the depth to a restrictive layer. Often, grasses (mostly dropseeds) and other shrubs, especially saltbush (Atriplex canescens), colonize dunes because of the greater availability of water there (Hennessy et al. 1985). Creosotebush (Larrea tridentata) may also dominate these shrublands where gravel content is high. Diagnosis: Mesquite is dominant. It often eventually forms low coppice dunes from 0.5-1 m high in the latter stages of development. Black grama and bunchgrasses may be absent and if present are often restricted to dunes. Snakeweed and a few bunchgrasses may occur in interdunes if an impermeable horizon has not been exposed. There is often evidence of eolian soil erosion including pedestalling, sand ripples and exposed caliche. Transition to bunchgrass grassland (8): In principle, it may be possible to kill mesquite, redistribute or add soil nutrients, and stabilize soil during periods favorable to the germination of bunchgrasses, perhaps in conjunction with seeding. The use of municipal biosolids may aid in restoring soil fertility (Walton et al. 2001). Information sources and theoretical background: Communities , states and transitions are derived largely from the literature including Buffington and Herbel (1965), Herbel et al. (1972), Gibbens and Beck (1987), Hennessy et al. (1983a), and Herbel and Gibbens (1996). Background information on the causes of transitions is presented in the Sandy site model. Much of the discussion below is the same as that presented in the Sandy model, and contrasts are drawn where information is available.